It makes me so sad when carefully crafted messages get buried in Slack history. People come and ask questions from different perspectives and get quality answers from developers. Of course it pays off immediately by educating those who asked, but history reuse is not possible after a week or so. Please consider http://slackarchive.io/ or an alternative, e.g. using some scripts found on GitHub to pull Slack history via API and dump it in plain HTML.
I agree, I hate the ephemeral nature of slack (part of why I prefer irc where logging is more normal and simpler). There was some discussion about using slackarchive but I'm not sure what came of it.
First Slack has cut off my Firefox 38 as "too old" and I could no longer post. I could still read chat logs via "Message Archives" feature (e.g. https://decred.slack.com/archives/general). Actually, even when chat was working I often read the logs via Archives because it did not drain my battery so fast. Now Slack seems to have removed the Message Archives feature. I've got a newer browser since then, but their attitude is concerning. It's tempting to think about Slack replacement.
I didn't realize slack was so aggressive about old versions of firefox. That is annoying. As for archives, I'm still infavor of using a third party archiver, but I don't really have the time to do it.